"What Sens. Manchin and Toomey are trying to do with this amendment is trying to make the Second Amendment stronger for the people who are law-abiding gun owners, but yet trying to keep guns out of the hands of folks who cannot handle them in a responsible way -- and have a record of that, a court-adjudicated record," he added.

As recently as Monday morning, Tester's office had said the senator was still reviewing the legislation. Montana's other senator, Max Baucus (D-Mont.), remains undecided.

With Tester's support, there are 49 Democrats and three Republicans who back the Manchin-Toomey legislation, which would strengthen and expand background checks for gun sales. Six Democrats and five Republicans are still undecided. No Democrats have said they will oppose it.

The compromise measure would require background checks for nearly all commercial gun sales, including online purchases, with exemptions for sales between family members as well as a provision to exempt short-term transfers between hunters. It would also explicitly ban a national gun registry and impose stiff penalties for anyone who attempts to create one.

on March 30, 1981, President Reagan and three others were shot and wounded in an assassination attempt by John Hinckley, Jr. outside the Washington Hilton Hotel in Washington, D.C. Reagan's press secretary, Jim Brady, was shot in the head.